Throughout the long history of submersible and submarine vessels a chief design and structural goal has been to provide a submarine or other underwater vessel having the capability of withstanding extreme external underwater pressure, which is directly related to functional depth operations, and also to be capable of maintaining structural integrity during rapid pressure changes due to necessary or accidental transitions of depth.
Collateral thereto is the necessity of ensuring safety to the occupants or crew within the submersible, and more particularly in the pressure hull thereof by virtue of the pressure hull strength, while yet providing a relative maximum amount of volume area for workspace and equipment space therein. In comparable manner, a submersible or submarine vessel must be capable of maximum speed of travel through the sea by virtue of streamline form, while yet maintaining high structural rigidity.
Development over many years, including the intense active engagements in two World Wars, have largely resulted in the development of submersibles and submarines having pressure hull configurations essentially of cylindrical or cigar-like form, having an external hydrodynamic form streamlined to suit surface operation, thus evolving an elongated, essentially linear, internal volume for habitation and machinery compartmentation, utilizing the inherent hoop strength of a small diameter and circular cross-section to adequately resist depth pressure. While this form has been and is universally employed for submarine form, the same are more suited for withstanding internal pressures, in an engineering sense, than an external hull collapsing pressure, the latter of which is a principal area of concern in submersibles and submarines.
Further advancements toward high underwater speeds and nuclear propulsion created the need for larger displacement hulls, significantly increasing the cylindrical hull diameters, and demanding the use of exotic and costly metallurgy to achieve the required hull strength, with attendant large increases in fabrication costs. This developmental change in submarines resulted in redefining the hydrodynamic form to suit a fully underwater operation, and introducing large single hull construction and shifting variable ballast systems away from amidships.
The demand for increased size has extended the overall hull length beyond the generally accepted length over diameter ration of six-to-one. The diameter is restricted by the draft limitations of harbors, and the increased structural requirements for enlarged diameter hulls. This ratio has proved unattainable throughout the twentieth century.
The need to limit growth in the vertical profile has in some instances required designers to employ two parallel laterally adjacent cylindrical pressure hulls to accommodate larger displacement, thereby shortening hull length and draft, and avoiding excessive wetted surfaces. Despite such efforts, the factors of limited depth capability and very high cost of construction are not alleviated. Further, the general cylindrical form of the pressure hull is not materially altered, even into very recent times. See, illustratively, the discussions of submarine design in Marine Technology, "SEAWOLF Design for Modular Construction", October, 1992 commencing at p. 199. It is noted, for example, that bulkheads in the SEAWOLF class submarines have diameters on the order of 40 feet, and an area of some 1,256 square feet each, with resultant serious and expensive fabrication and pressure resistance problems to be overcome in building the hull.
There is manifestly indicated a need for a pressure hull design that is far more structurally efficient, significantly less costly, and yet satisfy the volumetric demands of modern submarines. Further, and correlative thereto, there is practical and necessary interest in an improved submersible that would have a more shallow draft as compared to present constructions.